These studies are designed to determine the mechanism(s) by which prolactin has its actions in the mammary gland. Accordingly, we will pursue four projects in our research program. The first of these is a study of the action of prolactin on the activities of four enzymes in plasma membrane preparations and in isolated prolactin receptors of mammary glands; the enzymes to be studied include phospholipase A2, guanylate cyclase, adenylate cyclase and prostaglandin synthetase. The second project will involve continued studies on prolactin's actions on epithelial cells of mammary glands in monolayer cultures. A third project is designed to determine the mechanism by which prolactin has its profound and rapid action on ornithine decarboxylase activity in cultured mammary gland explants; we will specifically determine if the cyclic nucleotides or prostaglandins mediate this action of prolactin. And finally, we will carry out studies to determine the mechanism by which prolactin stimulates lipid metabolism in mammary cells.